The invention concerns a brake pressure setting device for automatic control of brake pressure change phases and brake pressure retention phases of a control system stabilizing the dynamic behavior of a vehicle and operating by action on the operational brake installation of the vehicle, e.g. an antilocking control system and/or a drive slip control system for a vehicle braking system wherein at least the wheel brakes of two driven vehicle wheels are combined into a static brake circuit, and wherein the wheel brakes can each be shut off individually, or jointly, from a main brake pipe by means of a brake pressure control valve designed as a solenoid valve. A connection control solenoid valve is provided in the main brake pipe of a static brake circuit between an outlet pressure space of a brake unit provided for brake pressure supply, operable by the driver and the driven wheel brakes. A pressure modulator, designed as a stepped cylinder which has two housing steps of different diameters, within which the movable boundaries of a control pressure space and a modulation chamber are formed by corresponding diameter flanges of a modulator piston is also provided. The modulation chamber is a smaller diameter chamber and is connected to the main brake pipe of the static brake circuit between the connection control valve and brake pressure control valves for the wheel brakes. A return spring is provided for urging the modulator piston into its end position associated with maximum volume of the modulation chamber. In addition, the control pressure space of the pressure modulator can be alternatively connected to the pressure outlet of an auxiliary pressure, source or alternatively relieved to its non-pressurized container by a function control valve arrangement to provide a system wherein brake pressure changes can be controlled. A position indicator generates an electrical output signal characteristic of the position of the modulator piston as an input to an electronic control apparatus, which in turn generates output signals necessary for correct triggering of the connection control valve, the function control valve arrangement and the brake pressure control valves by processing the output signals of the position indicator and the output signals of wheel rotational speed sensors, which generate electrical output signals characteristic of the dynamic behavior of the vehicle wheels.
Such a brake pressure setting device in association with an antilocking system for a road vehicle is the subject matter of the applicant's own, older, German patent No. DE-3,723,875.2 which corresponds to pending U.S. application No. 07/220,168 filed July 18, 1988 now U.S. Pat. No. 4,877,296.
The antilocking system described therein is for a road vehicle with front axle/rear axle brake circuit subdivision and with a rear axle drive. The brake circuit associated with the driven rear wheels is designed as a static brake circuit with branch brake pipes, each leading from a main brake pipe to one of the rear wheel brakes. These branch brake pipes can be shut off, either individually or jointly, from the main brake pipe by a solenoid brake pressure control valve. The main brake pipe can itself be shut off from the outlet pressure space, associated with the static brake circuit of a brake unit provided for supplying brake pressure from a tandem main cylinder, operable by the driver and of conventional construction, by means of a solenoid connection control valve. Also provided is a Pressure modulator, designed as a stepped cylinder, which has two bore steps of different diameters connected to one another by means of a housing step. This cylinder defines the movable boundaries of a control pressure space and a modulation chamber, formed by flanges of corresponding diameters on a modulator piston. The control pressure space and the modulation chamber have boundaries fixed relative to the housing and formed by end walls of the modulator housing. The smaller diameter chamber is used as the modulator chamber and this is connected to the main brake pipe of the static brake circuit between the connection control valve and the brake pressure control valves. The pressure modulator is provided with a powerful return spring which attempts to force the modulator piston into its end position associated with maximum volume of the modulation chamber. The control pressure space, whose movable boundary is formed by the larger piston flange, can be alternatively connected, by means of a function control valve arrangement, to the pressure outlet of an auxiliary pressure source, by which means the modulator piston can be displaced in the direction of reducing the volume of the modulation chamber, or alternatively it can be connected to the non-pressurized sump tank of the auxiliary pressure source wherein the piston experiences a displacement in the direction of increasing the volume of the modulation chamber. This makes it possible to control brake pressure build-up and brake pressure reduction phases of the antilocking control system. Also provided is a position indicator which creates electrical output signals characteristic of the position of the modulator piston. One wheel rotational speed sensor is allocated to each of the driven vehicle wheels and the non-driven vehicle wheels for generating electrical output signals which, in level and/or frequency, are a measure of the wheel peripheral speeds of the vehicle wheels and, in terms of their changes, also contain information on the acceleration or deceleration of the vehicle wheels. These output signals of the wheel rotational speed sensors and the output signals of the position indicator are processed by an electronic control unit to provide output signals by means of which the solenoid valves of the brake pressure control device can be triggered in the order and combination corresponding to the control objective.
The patent No. De-3,723,875 mentioned reveals no measures of any type by which the brake pressure setting device could also be used in an analogous manner for a drive slip control system operating on the principle of retarding a vehicle wheel tending to spin, by reactivating its wheel brake to such an extent that a stable dynamic behavior of the vehicle is achieved even during acceleration. On the basis of the brake pressure setting device therein described, however, it would be desirable to achieve a drive slip control function in that, as soon as a spin tendency is recognizable on one of the driven vehicle wheels, the modulator piston is brought into its end position associated with maximum volume of the modulation chamber--by subjecting the control pressure space to pressure--in order to force brake fluid into the wheel brake of the vehicle wheel tending to spin, while the modulation chamber is simultaneously shut off from the outlet pressure space of the brake unit. This appears quite possible by analogous control of the function control valve arrangement and use of the brake pressure control valves, again assuming an appropriate design of the electronic control apparatus.
In a combined antilocking and drive slip control system realized in such a way however, "different" preparation phases would be necessary for the particular control operation, i.e. the modulator piston would have to have been previously brought into its end position associated with minimum volume for the anti-locking control operation, whereas it would first have to be brought into the end position associated with maximum volume of the modulation chamber as the initial position for the control in preparation for drive slip control operation.
In a traffic situation in which the brake pressure setting device has been prepared for drive slip control operation, and where the driver must unexpectedly brake, this would have the disadvantage that the "pole reversal" of the brake pressure setting device to a possible antilocking control operation, which now becomes necessary, would demand a relatively large period of time because the modulator piston must first be brought into its end position associated with minimum volume of the modulation chamber in order to make braking possible while using the antilocking control function.
Although it is conceivable that rapid response of the brake installation could be ensured in such a situation by initially doing without the antilocking control function, this would indubitably be very dangerous in the traffic situation assumed as an example, because if the road conditions already require drive slip control, it may be assumed with high probability that the antilocking control function will be necessary if abrupt braking occurs in such a situation.
The object of the instant invention is therefore to improve a brake pressure setting device of the aforementioned type in such a way that it can be switched over from drive slip control to antilocking control, as required, very much more rapidly.
The invention achieves this object by connecting a mechanical controlled valve between the outlet pressure space associated with the brake circuit of the driven vehicle wheels and the connection control valve. The modulation chamber of the pressure modulator is also connected via an outlet non-return valve to a valve chamber of this mechanically controlled valve, which can be driven into an open position by higher pressure in the modulation chamber relative to the pressure in the valve chamber. The mechanically controlled valve is designed as a displacement-controlled valve with a valve body displaced in the direction of its shut-off position for causing the outlet pressure space of the brake unit to be shut off relative to the valve chamber, by displacement of the modulator piston taking place, so as to reduce the volume of the modulation chamber. From an intermediate position of the piston, corresponding to a volume of the modulation chamber of between 75% and 50% of its maximum volume, the mechanically controlled valve arrives at its shut-off position. The mechanically controlled valve is designed as a pressure-controlled valve which, at least when the piston is located in the intermediate position mentioned, or in a position corresponding to a larger volume of the modulation chamber, can be driven into its open position by pressure in the outlet pressure space of the brake unit which is higher than the pressure in the valve chamber. Upon the appearance of a locking tendency of the wheels by the wheel brakes of the static brake circuit, the electronic control apparatus generates output signals operating the antilocking control operation. By means of these signals, the modulator piston is displaced into its intermediate position and subsequently, the connection control valve is switched into its shut-off position, while the brake pressure control valve of a vehicle wheel not tending to lock is driven into its shut-off position, and the function control valve arrangement is switched into a functional position causing pressure relief of the control pressure space. At the latest, on the appearance of a spin tendency on one of the driven vehicle wheels, the electronic control apparatus generates an output signal combination by means of which the drive slip control operation is controlled. Here the function control valve arrangement is driven into a functional position causing connection of the control pressure space to the pressure outlet of the auxiliary pressure source, while the connection control valve is driven into its shut-off position, and the brake pressure control valve of the vehicle wheel not tending to spin, is also driven into its shut-off position. In normal braking (driving or braking operations not subject to a control system) the connection control valve and the brake pressure control valves are held in their through-flow positions and the modulator piston, monitored by means of the position indicator, is held in a position which at least corresponds to the volume of the modulation chamber associated with its intermediate position.
According to this concept of the invention, the preparatory position of the modulator piston, provided for both drive slip control and antilocking control, is a position between its possible end positions associated with minimum and maximum volumes of the modulation chamber. From this intermediate position, the modulator piston can then be displaced "immediately", i.e. without further preparatory motion, either so as to increase the volume of the modulation chamber, as is necessary for an introductory pressure reduction phase of the antilocking control, or alternatively to decrease the volume of the modulation chamber, as is necessary for an introductory brake pressure build-up phase in the case of a drive slip control. Depending on which type of control is necessary, the pressure modulator is so dimensioned overall and the intermediate position of the modulator piston is so selected, that marked brake pressure changes in the particular change direction necessary are possible by means of the pressure changing strokes possible with this arrangement. If a single stroke of the modulator piston is not sufficient to achieve the brake pressure change necessary for the particular type of control, the modulator piston is returned to its initial position and the particular pressure change stroke is repeated. Here the pressure modulator operates like a hydraulically driven return pump in the event of a repetition of brake pressure reduction strokes. The piston positions can then be monitored and followed, as is necessary in this case, in a simple manner by means of the position indicator output signal. An essential advantage of the invention is then seen in the fact that the control responds immediately in every case and in the necessary control direction, i.e. there are no "dead periods" which would otherwise have to be accepted when switching over from one type of control to the other. A reduction in the sensitivity of the control caused by a possible necessary repetitive operation of pressure change strokes of the modulator piston, the reduction in sensitivity ultimately being the result of the time required for this purpose, is trivial at the most. This time can be limited to a minimum because the relevant piston displacements can be held to the necessary extent by appropriate assessment of the output signals of the wheel rotational speed sensors and the position indicator output signal. For example, where a single brake pressure change stroke is not sufficient to prepare for a further brake pressure change stroke, the modulator piston is only displaced as far in the direction of its initial position as is necessary to be able to achieve a sufficient change in the particular direction required during the next pressure change stroke.
In the preferred design of the brake pressure setting device the modulator piston is held in the intermediate position provided both as the initial position for a pressure reduction phase of the antilocking control and as the initial position for a pressure build-up phase of the drive slip control, both in normal driving operation and also in the case of normal braking operation not subject to an antilocking control. The setting device is, as it were, permanently prepared for a control operation which to this extent ensures optimum sensitivity of the particular control operation.
It is desireable if the difference in the volumes of the modulation chamber and the pressure modulator, between their minimum and maximum values, corresponds to between 20% and 50% of the volume of that quantity of brake fluid which has to be forced into the brake circuit of the driven vehicle wheels in order to generate the maximum possible brake pressure in the wheel brakes and that the modulator piston, in the intermediate position used as the initial position for the antilocking control and the drive slip control, takes up a position from which the volume of the modulation chamber can be increased by at least 10% of that volume of brake fluid which is accepted by the static brake circuit when the pressure is built up to the maximum value for which the brake installation is designed. With these dimensional relationships for the pressure modulator of the brake pressure setting device, a single brake pressure change stroke of the modulator piston will suffice to adequately stabilize the dynamic behavior of the vehicle in an overwhelming majority of the braking or acceleration situations requiring control.
It is advantageous if the mechanically controlled valve is designed as a seat valve whose valve body is supported by means of a preloaded return spring, which can be compressed in the axial direction, and wherein an actuation element penetrates axially displaceably and in a pressure-tight manner through a hole in a housing wall separating the modulation chamber of the pressure modulator from the valve chamber. Here relative motions of the valve body and the actuation element are limited by stops to a deflection corresponding to the stroke of the modulator piston from its initial position into the end position associated with minimum modulation chamber volume. The actuation element arrives in axially supporting contact with the modulator piston, at the latest, when the latter has executed a small fraction of its stroke from its initial position thus causing the reductions to the modulation chamber volume. The maximum opening stroke of the seat valve corresponds to only a small fraction of between 1/20 and 1/10 of the maximum possible stroke of the modulator piston in the direction of decreasing the volume of the modulation chamber. The mechanically controlled valve is designed as a ball/seat valve with a ball-ended valve body, located on the outside of a bottom of a cylindrical pan-shaped retention sleeve. A conical valve seat, designed as an opening, is on the valve chamber side of an inlet duct. The end of the actuation element is located within the valve chamber and is designed as a radial flange on the housing side facing towards the valve body and supports the return spring. Its side facing towards the modulation chamber has behind it, in its outer edge region, a contact flange, pointing radially inwards of the sleeve carrying the valve body. This provides the necessary construction of a mechanically controlled valve which can be used for the control of brake pressure build-up phases of the drive slip control. This valve shuts off the brake unit from the brake circuit of the driven vehicle wheels when the drive slip control responds.
By having the function control valve arrangement include a first 2/2-way solenoid valve and a second 2/2-way solenoid valve, whose basic positions are their shut-off positions and whose actuated positions are their through-flow positions and with the first solenoid valve being connected between a control connection of the pressure modulator and a high pressure outlet of the auxiliary pressure source and the second solenoid valve being connected between the control connection and a non-pressurized sump container of the auxiliary pressure source provides a simple and functionally reliable configuration of the function control valve arrangement provided for the stroke control of the modulator piston.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.